1. Field of the Invention
This invention relates generally to electromagnetic-mechanical transducers for driving a load or being driven by a prime mover and more particularly relates to reciprocating linear motors and reciprocating linear alternators
2. Description of the Related Art
Reciprocating linear alternators are used for generating electrical power when driven by any of a variety of prime movers, including Stirling engines, and linear motors are used for driving a variety of mechanical loads when powered by an AC power source Like rotating motors, generators and alternators, such linear motors and linear alternators are essentially identical in that they both have the same basic components and the difference is the way they are connected and operated. Therefore, they are referred to collectively as linear motor/alternators.
The prior art includes a basic linear motor/alternator of the type illustrated in FIG. 3 and also described in U.S. Pat. No. 4,602,174, which is hereby incorporated by reference, and U.S. Pat. No. 4,623,808 which is hereby incorporated by reference. Although a linear motor/alternator can be constructed in a variety of configurations known in the prior art, the preferred configuration is an axisymmetric configuration in which an actuator carrying permanent magnets reciprocates along an axis of reciprocation within an armature. The permanent magnets are mounted to the reciprocating actuator in a cylindrical arrangement that is concentric with the axis. The main part of the armature core and the armature coil or winding are also mounted in a concentric cylindrical arrangement around the magnets and are mounted to a frame so they remain stationary. The remainder of the core completes the flux loop formed by the core and is also mounted to the stationary frame in a cylindrical arrangement. That remainder of the core is spaced inwardly from the principal part of the core to form linearly aligned gaps in the high reluctance flux path of the core. The linearly aligned gaps are parallel to the axis The magnets (or a circular magnet) reciprocate within the gaps formed in the core. The armature can be a series of individual armatures spaced around a circle in the cylindrical configuration or it can be a circular armature with a circular coil in a circular slot. Similarly, the magnets can be discrete magnets placed side by side in a cylindrical arrangement or a circular magnet. As another alternative known in the art, the coil can be wound around a leg of the core. In all these alternative configurations, time-varying magnetic flux in the core induces a current in the coil and current in the coil induces magnetic flux in the core. These configurations are illustrated and described in the above two cited patents and in U.S. Pat. No. 5,148,066 which is herein incorporated by reference.
FIG. 3 illustrates the basic components of a prior art linear motor/alternator. For an axisymmetric linear motor/alternator, FIG. 3 is a cross sectional view in a plane on which the axis of reciprocation 10 lies and along one radial from that axis. The cross sectional view of these basic components along the opposite radial and in the same plane is a mirror image of FIG. 3, which is therefore not duplicated.
Referring to FIG. 3, an armature 12 has an associated armature coil 14 and an associated core 16. The armature coil 14 is wound in a circular configuration that is concentric with the axis 10. The core 16 forms a low reluctance magnetic flux loop that consists of a u-shaped principal part 18 and a remaining part 20, both constructed of laminations of iron or other high permeability material as well known in the art. The core loop has a pair of spaced gaps 22 and 24 that are parallel to the axis 10 of reciprocation and separated from each other by an armature winding slot 26. Each of the gaps 22 and 24 are defined by two opposed pole faces which are pole faces 28 and 30 defining gap 22 and pole faces 32 and 34 defining gap 24.
The gaps 22 and 24 are linearly aligned along a gap path parallel to the axis so that a field magnet 36 that is associated with the armature 12 can reciprocate in an axial direction within the gaps 22 and 24. The field magnet 36 is mounted to a reciprocatable actuator 38 which carries all magnets so that the magnets reciprocate within the gap path of gaps 22 and 24. The field magnet 36 is polarized across the gaps 22 and 24 preferably perpendicular to the pole faces 28-34 as shown by the arrow drawn on the center of the magnet 36. The actuator 38 is drivingly connected to a prime mover or load 40, depending upon whether the linear motor/alternator of FIG. 3 is used as a linear alternator or as a linear motor. As the magnet 36 reciprocates to alternately enter between the gap 22 and the gap 24, the magnetic flux in the core resulting from the magnet 36 alternately reverses. Because the magnetic flux path through the core extends through the armature coil and varies with time, an EMF is induced into the coil and current in the coil generates a magnetic flux in the core that applies a force to the magnets in the manner well known to those skilled in the art.
The linear motor/alternator structures described above give suitable performance for many applications of a linear motor/alternator. However, for some applications, it is desirable that a spring force be applied to the reciprocating actuator. For example, if the linear motor/alternator is driven by a free-piston Stirling engine or drives a free-piston Stirling cooler, a spring force that is applied in a direction toward centering the actuator is desirable for maintaining the axial mean position of such a free-piston machine at a selected center position because such Stirling machines have a tendency for their mean position to drift away from the nominal centered position. As another example, it is sometimes desirable for the actuator of a linear motor/alternator and its load or prime mover to reciprocate in a resonant system, which requires a spring. As yet another example, if the reciprocation of the actuator and its load or prime mover has a component that is vertical, it is sometimes desirable to provide a centering spring force on the actuator to resist the force of gravity and prevent the actuator from moving to the lowest limit of excursion from its mean position.
Mechanical springs can and have been used for this purpose. However, mechanical springs have some detrimental characteristics. U.S. Pat. No. 5,148,066, cited above, discloses a way to introduce a magnetic spring force into the linear motor/alternator. As described in that patent, a pair of smaller secondary magnets are placed on opposite sides of the main magnet and are polarized oppositely to the main magnet. These secondary magnets cause a centering force to be exerted on the reciprocating actuator whenever one of the secondary magnets extends outwardly from between the pole faces that define one of their two gaps. Because the centering spring force is applied to the actuator only when a secondary magnet moves out of a gap, the secondary magnets extend from the main magnet all the way to the outer edge of their respective gaps. That way there is no dead zone, around the mean centered position, in which there is no spring force applied to the actuator tending to return the actuator to its mean position. The combination of the three magnets illustrated and described in U.S. Pat. No. 5,148,066 extend from the outer edge of one gap to the opposite outer edge of the other gap. When the actuator is moved, one secondary magnet is displaced out of the gap between the pole faces and causes a force that is applied in a direction toward centering the magnet and that has a magnitude that is proportional to its displacement out of the gap.
However, problems result from the fact that one of the secondary magnets is essentially always moving outside of the gap so that a spring force will be applied to the actuator without power production. The purpose, object and feature of the present invention is to eliminate those problems. The first problem is that the contribution of the secondary magnets to the generation of electric power in an alternator or to the application of a motor driving power in a motor is diminished the more the secondary magnets extend out from a gap into the air. The reason is that the air has a very low permeability and consequently the flux in the core from the secondary magnets is relatively small. The second problem arises because the alternating reciprocation of the secondary magnets from within a gap to a position extending out from its gap creates a time-varying magnetic fringe field outside the pole faces. This alternating magnetic fringe field is coupled to surrounding ferromagnetic materials and induces eddy currents in those ferromagnetic materials producing resistive electrical losses. Additionally, the same alternating fringe fields are coupled to nearby conductors which interferes with electrical currents in those conductors.